Speed reducer devices have been applied to robot techniques in which various component parts are transferred during manufacturing processes (e.g., producing digital versatile disks (DVD)). In the speed reducer devices, upon implementing the power transmission, an output shaft is reduced at its speed by driving the input shaft which is connected to a brachiate arm. The speed reducer device is known as a worm wheel device which has a transmission mechanism in which balls are provided between a worm screw and a wheel (referred to e.g., Japanese Laid-open Patent Application No. 6-109081). The worm screw has a series of helical grooves defined as thread helices, and the wheel has semi-circular cavities in which the respective rolling balls are provided.
The worm screw transmits its rotational movement to the wheel by way of the rolling balls, thereby avoiding a backlash played between the worm screw and the wheel so as to insure a low friction and high transmissibility therebetween.
In the worm wheel device disclosed by the Japanese Laid-open Patent Application No. 6-109081, the helical grooves of the worm screw represent the cammed grooves into which the semi-spherical portions of the respective balls are always fit in a surface-to-surface contact. Upon transmitting the rotational movement of the worm screw to the wheel while meshing the balls with the cammed grooves, the cammed grooves slide its concave surface along the spherical surface of the balls so as to implement a torque transmission between the worm screw and the wheel.
It is disadvantageous to slide the cammed grooves along the balls in the surface-to-surface contact from the point of inducing the friction between the cammed grooves and the balls. In order to implement a point-to-point contact instead of the surface-to-surface contact, it is necessary to provide the input shaft with streaks of cammed rib instead of the cammed grooves. In the ribbed cam structure in which the ribbed cam has a jutted portion, the cammed rib comes to engage with the balls to establish a pushable force in association with the rotational movement of the input shaft. This causes to induce a backlash played at an engagement surface between the cammed rib and the balls due to the mechanical reason.
Therefore, it is an object of the invention to overcome the above drawbacks, and provide a ball type speed reducer device which is capable of eliminating a backlash when the cammed rib structure is adopted to engage cammed ribs (jutted streaks) with balls in a point-to-point contact.
It is other object of the invention to provide a ball type speed reducer device which is capable of avoiding a meshing noise when engaging the cammed ribs with the balls so as to implement a low noise operation, while at the same time, improving a torque transmissibility against a follower wheel to insure a smooth rotational movement with a higher precision.